Study On Yield Strength Prediction Model For Hot Stamping Of Al-Zn Alloy

With the increasingly strict energy conservation and environmental protection regulations,lightweight technology has been paid attention to by the automotive industry.Aluminum alloys are considered to be potential materials for automotive light weighting due to their excellent mechanical and formability properties.In view of the problem of poor formability of aluminum alloys at room temperature,hot stamping technology has been widely used as an advanced manufacturing technology.In order to avoid the long production cycle of traditional hot stamping,a pre-aged hardening warm forming(PHF)process will be studied in this thesis.This thesis will take Al-Zn alloy as the research object,will explore the hot deformation behavior under different hot stamping process parameters,will analyze the properties and microstructure evolution of hot stamping,and will establish a yield strength prediction model for hot stamping of Al-Zn alloy.With MMS200 thermal simulator,the Al-Zn alloys under different deformation temperature(180～220℃),strain rate(0.01～1s-1),pre-aging time(3～24h)and soaking time(1～5min)were studied.The thermal tensile deformation behavior was analyzed,and the influence of different process parameters on the mechanical properties was analyzed.Based on its rheological behavior,a strain-compensated constitutive model with Z parameters and a PSO-BP artificial neural network constitutive model were established.The statistical indexes R and AARE of the two were 0.961,8.761% and 0.9935,2.51%,respectively.The mechanical properties and microstructure evolution of Al-Zn alloy during hot stamping were studied by room temperature tensile,OM,TEM,XRD and other tests.The yield strength and tensile strength of the deformed samples showed a trend of increasing first and then decreasing,and reached the maximum after 10 min of heat preservation.The OM analysis results showed that the grain size changes little before and after hot stamping.TEM analysis showed that the precipitation phase in the pre-aging sample is mainly GPII region,the transformation from GPII zones to theη ’phase occurs during the heat preservation forming process,and the main precipitation phase after hot stamping was theη ’ phase.The DSC test showed that the phase transition process in the heat preservation forming process is similar.In order to make the GP region as far as possible to phase transition,the most suitable heat preservation forming temperature was 200±20℃,and the phase activation energy under different pre-aging time was 104.53～121.07 k J/mol.The established evolution model of Al-Zn alloy precipitates showed that with the increase of holding time,the size and volume fraction of precipitates both show an upward trend.The established model was verified by SAXS test,and the results showed that the error of the two was within 3.5% ～5.3% and 5.2%～7.4%.The strengthening mechanisms of the Al-Zn alloy hot stamping yield strength prediction model were analyzed and calculated,and the yield strength of the specimens soaked at 200℃/5～25min was predicted and verified.The results showed that the error was within 2.28%～4.35%.The finite element simulation of the Al-Zn alloy automotive B-pillar hot stamping process was carried out by DYNAFORM software,and the reliability of the process was ensured through the analysis of the simulation results and the verification of the hot stamping test.